Development of Oligonucleotides for Therapeutic Applications
There is much interest in the medical uses of nucleic acids. For example, antisense, ribozymes, aptamer and RNA interference (RNAi) technologies are all being developed for potential therapeutic applications. The design of nucleic acids, particularly oligonucleotides, for in vivo delivery requires consideration of various factors including binding strength, target specificity, serum stability, resistance to nucleases and cellular uptake. A number of approaches have been proposed in order to produce oligonucleotides that have characteristics suitable for in vivo use, such as modified backbone chemistry, formulation in delivery vehicles and conjugation to various other moieties. Therapeutic oligonucleotides with characteristics suitable for systemic delivery would be particularly beneficial.
Oligonucleotides with modified chemical backbones are reviewed in Micklefield, Backbone modification of nucleic acids: synthesis, structure and therapeutic applications, Curr. Med. Chem., 8(10):1157-79, 2001 and Lyer et al., Modified oligonucleotides-synthesis, properties and applications, Curr. Opin. Mol. Ther., 1(3): 344-358, 1999.
Examples of modified backbone chemistries include:                peptide nucleic acids (PNAs) (see Nielsen, Methods Mol. Biol., 208:3-26, 2002),        locked nucleic acids (LNAs) (see Petersen & Wengel, Trends Biotechnol., 21(2):74-81, 2003),        phosphorothioates (see Eckstein, Antisense Nucleic Acid Drug Dev., 10(2):117-21, 2000),        methylphosphonates (see Thiviyanathan et al., Biochemistry, 41(3):827-38, 2002),        phosphoramidates (see Gryaznov, Biochem. Biophys. Acta, 1489(1):131-40, 1999; Pruzan et al., Nucleic Acids Res., 30(2):559-68, 2002), and        thiophosphoramidates (see Gryaznov et al., Nucleosides Nucleotides Nucleic Acids, 20(4-7):401-10, 2001; Herbert et al., Oncogene, 21(4):638-42, 2002).        
Each of these types of oligonucleotides has reported advantages and disadvantages. For example, peptide nucleic acids (PNAs) display good nuclease resistance and binding strength, but have reduced cellular uptake in test cultures; phosphorothioates display good nuclease resistance and solubility, but are typically synthesized as P-chiral mixtures and display several sequence-non-specific biological effects; methylphosphonates display good nuclease resistance and cellular uptake, but are also typically synthesized as P-chiral mixtures and have reduced duplex stability. The N3′→P5′ phosphoramidate internucleoside linkages are reported to display favorable binding properties, nuclease resistance, and solubility (Gryaznov and Letsinger, Nucleic Acids Research, 20:3403-3409, 1992; Chen et al., Nucleic Acids Research, 23:2661-2668, 1995; Gryaznov et al., Proc. Natl. Acad. Sci., 92:5798-5802, 1995; Skorski et al., Proc. Natl. Acad. Sci., 94:3966-3971, 1997). However, they also show increased acid lability relative to the natural phosphodiester counterparts (Gryaznov et al., Nucleic Acids Research, 24:1508-1514, 1996). Acid stability of an oligonucleotide is an important quality given the desire to use oligonucleotide agents as oral therapeutics. The addition of a sulfur atom to the backbone in N3′→P5′ thiophosphoramidate oligonucleotides provides enhanced acid stability.
As with many other therapeutic compounds, the polyanionic nature of oligonucleotides reduces the ability of the compound to cross lipid membranes, limiting the efficiency of cellular uptake. Various solutions have been proposed for increasing the cellular uptake of therapeutic agents, including formulation in liposomes (for reviews, see Pedroso de Lima et al., Curr Med Chem, 10(14):1221-1231, 2003 and Miller, Curr Med Chem., 10(14):1195-211, 2003) and conjugation with a lipophilic moiety. Examples of the latter approach include: U.S. Pat. No. 5,411,947 (Method of converting a drug to an orally available form by covalently bonding a lipid to the drug); U.S. Pat. No. 6,448,392 (Lipid derivatives of antiviral nucleosides: liposomal incorporation and method of use); U.S. Pat. No. 5,420,330 (Lipo-phosphoramidites); U.S. Pat. No. 5,763,208 (Oligonucleotides and their analogs capable of passive cell membrane permeation); Gryaznov & Lloyd, Nucleic Acids Research, 21:5909-5915, 1993 (Cholesterol-conjugated oligonucleotides); U.S. Pat. No. 5,416,203 (Steroid modified oligonucleotides); WO 90/10448 (Covalent conjugates of lipid and oligonucleotide); Gerster et al., Analytical Biochemistry, 262:177-184 (1998) (Quantitative analysis of modified antisense oligonucleotides in biological fluids using cationic nanoparticles for solid-phase extraction); Bennett et al., Mol. Pharmacol., 41:1023-1033 (1992) (Cationic lipids enhance cellular uptake and activity of phophorothioate antisense oligonucleotides); Manoharan et al., Antisense and Nucleic Acid Drug Dev., 12:103-128 (2002) (Oligonucleotide conjugates as potential antisense drugs with improved uptake, biodistribution, targeted delivery and mechanism of action); and Fiedler et al., Langenbeck's Arch. Surg., 383:269-275 (1998) (Growth inhibition of pancreatic tumor cells by modified antisense oligodeoxynucleotides).
Telomerase as a Therapeutic Target
Telomerase is a ribonucleoprotein that catalyzes the addition of telomeric repeat sequences to chromosome ends. See Blackburn, 1992, Ann. Rev. Biochem., 61:113-129. There is an extensive body of literature describing the connection between telomeres, telomerase, cellular senescence and cancer (for a general review, see Oncogene, volume 21, January 2002, which is an entire issue of the journal focused on telomerase). Telomerase has therefore been identified as an excellent target for cancer therapeutic agents (see Lichsteiner et al., Annals New York Acad. Sci., 886:1-11, 1999).
Genes encoding both the protein and RNA components of human telomerase have been cloned and sequenced (see U.S. Pat. Nos. 6,261,836 and 5,583,016, respectively) and much effort has been spent in the search for telomerase inhibitors. Telomerase inhibitors identified to date include small molecule compounds and oligonucleotides. Various publications describe the use of oligonucleotides to inhibit telomerase, either targeted against the mRNA encoding the telomerase protein component (the human form of which is known as human telomerase reverse transcriptase or hTERT) or the RNA component of the telomerase holoenzyme (the human form of which is known as human telomerase RNA or hTR). Oligonucleotides that are targeted to the hTERT mRNA are generally believed to act as conventional antisense drugs in that they bind to the mRNA, resulting in destruction of the mRNA, and thereby preventing production of the hTERT protein (see, for example, U.S. Pat. No. 6,444,650). Certain oligonucleotides that are targeted to hTR are designed to bind to hTR molecules present within the telomerase holoenzyme, and thereby disrupt enzyme function (see, for example, U.S. Pat. No. 6,548,298). Examples of publications describing various oligonucleotides designed to reduce or eliminate telomerase activity include:
U.S. Pat. No. 6,444,650 (Antisense compositions for detecting and inhibiting telomerase reverse transcriptase);
U.S. Pat. No. 6,331,399 (Antisense inhibition of tert expression);
U.S. Pat. No. 6,548,298 (Mammalian telomerase);
Van Janta-Lipinski et al., Nucleosides Nucleotides, 18(6-7):1719-20, 1999 (Protein and RNA of human telomerase as targets for modified oligonucleotides);
Gryaznov et al., Nucleosides Nucleotides Nucleic Acids, 20: 401-410, 2001 (Telomerase inhibitors-oligonucleotide phosphoramidates as potential therapeutic agents);
Herbert et al., Oncogene, 21(4):638-42, 2002 (Oligonucleotide N3′→P5′ phosphoramidates as efficient telomerase inhibitors);
Pruzan et al., Nucleic Acids Research, 30(2):559-568, 2002 (Allosteric inhibitors of telomerase: oligonucleotide N3′-P5′ phosphoramidates);
PCT publication WO 01/18015 (Oligonucleotide N3′-P5′ thiophosphoramidates: their synthesis and use); and
Asai et al., Cancer Research, 63:3931-3939, 2003 (A novel telomerase template antagonist (GRN163) as a potential anticancer agent).